Generally in a NOx removal device, a reducing agent is sprayed into exhaust gas in a smoke path, and nitrogen oxides contained in the exhaust gas are removed in the form of nitrogen and water by means of chemical reactions with a catalyst. The reducing agent sprayed into the exhaust gas evaporates, and the temperature of the exhaust gas is reduced at this time due to the latent heat of evaporation.
The spraying conditions of the reducing agent are adjusted by measuring the temperature of the exhaust gas into which the reducing agent is sprayed during a trial operation of the boiler plant. The NOx removal device is employed in the actual operation of the boiler plant in the same state as the state thereof after the spraying conditions of the reducing agent are adjusted during the trial operation of the boiler plant.
In the disclosures of Japanese Unexamined Patent Application, Publication No. Hei 8-233668 and Japanese Unexamined Patent Application, Publication No. Hei 6-221932, the temperature of the exhaust gas into which the reducing agent is sprayed is measured optically.
For controlling the spraying conditions of the reducing agent during the actual operation of the boiler plant, flow-rate control of the reducing agent sprayed into the exhaust gas is performed by measuring the nitrogen oxide concentration and reducing agent concentration (for example, Japanese Unexamined Patent Application, Publication No. Hei 7-60066 and Japanese Unexamined Patent Application, Publication No. 2003-290630).
However, with the inventions disclosed in Japanese Unexamined Patent Application, Publication No. Hei 7-60066 and Japanese Unexamined Patent Application, Publication No. 2003-290630, there is a problem in that the concentration distribution of the reducing agent does not reach a desired concentration distribution when a nozzle, etc. is blocked, even in the case in which the spraying level of the reducing agent is kept constant, and that the efficiency of the NOx removal devices consequently becomes inferior.